We don't actually have bid day shirts since we don't participate in formal recruitment... but i have seen the other bid day shirts and rush shirts from campus:
last year i saw phi sig with the greek goddess thing mentioned before.
gamma phi had a vegas theme with "i hit the jackpot at gamma phi's bid day" or something to that affect.
AXO did something one year where it said "if you had three wishes, what would the other one be?"
ZBT and Sigma Pi has done the whole Mastercard/Priceless theme
We did the VW theme, a hawaiian theme with "best friends are we, my sisters and me" on the back, and then Chi Delta on fire... not too original but the shirt was tight.
I think this semester we are doing a "cheers" theme where i think they wanna say "you wanna go where all your sisters know your name."
Dave Matthews Band songs seem to be good sources for themes (The best of what's around...)
Hope that helps